Often, as I walk my three-year-old up the stairs to bed, he asks, “Is this it, mommy? Is the day over?”

His eyes cloud over with disbelief when I say yes and I can see him trying to mentally walk backward through his day. I remind him that we started with breakfast and he nods. When I talk him through our morning trip to the park and the sandwiches we shared for lunch he smiles. He giggles as I describe how he made his brother laugh for the first time that afternoon and squares his shoulders with pride when I note that the tower he built as I cooked dinner was his tallest yet. “And now we’re here,” I say. “The sun has gone to the other side of the earth and it’s time for us to rest.”

As his disbelief in the passage of time shifts to an understanding that today really is over, he turns his deep hazel eyes to meet mine. “But I want today to keep going,” he says. “Can we make it last just a little bit longer? Maybe we can read a book together, or just snuggle?”

Though my concept of time is sturdier than my boy’s, I know exactly how he feels. Earlier this year, as he blew out three candles on his cake, I wondered how the time had passed so quickly. After I tucked him into bed that night, I dug out his memory envelope, the working mom’s version of a baby book. As I dumped it on the bed, I took note first of the black and white ultrasound picture, curled already on the edges, that we’d taken home from my first prenatal appointment. That afternoon we’d been scared, then elated, when we saw the tiny dot of our baby-to-be and heard the rapid thumping that had been missing the last time we’d visited the obstetrician just a few months before. As I looked at the date stamped on the upper right corner I nodded, thinking, “That really was almost four years ago.”

As I reach into the pile of my boy’s artifacts, my fingers close on his hospital bracelet, the one he wore at four days old when I carried him, yellow and lethargic, into children’s ward. As I handed him over to the nurses, I cried, ashamed of just how quickly I’d failed as a mother. As the lights and my milk healed him, I began to understand our relationship. By his third day when the nurses referenced a person called “mom,” I understood that they were talking about me. When we returned home and I snipped the hospital bracelet from his delicate wrist, I smiled. He was mine. Three years ago. I guess that sounds about right.

Amid the bracelets and ultrasound images, the tiny baggies of blond curls, the inked footprints and the finger paintings, there are pictures. Snapshots of moments that never made it into photo albums, images of a baby boy, smiling, laughing, standing for the first time. Snapshots of a toddler wearing a too-big backpack next to the preschool welcome sign, of a boy playing soccer, and of a boy mixing muffins with me in the kitchen. As I thumb through the pictures, across time and out of order, I see winters and springs and summers and falls. An infant, then a baby, then a toddler, then a boy. I square my shoulders with pride. My boy has grown so much.

When I reach backward in my mind to the earliest moments of my boy’s life, I suppose it makes sense that it’s been almost four years since he came into being. In the day-to-day, though, as I watch him play or read or dream, I can’t believe that this part of his life is already passing. I catch my breath as I think backward. His babyhood is gone. Evaporated. Passed as quickly as the day has to my boy.

He’s still young, I know. There are far more firsts to come than firsts that have passed but, already, I feel a disbelief in the quickness of time and the child growing right before my eyes. When he turned one, I gasped. At two, I did the same and, judging by the way my mother sighed my age into my ear as she hugged me on my last birthday, I assume I’ll be in disbelief as long as I’m alive.

And when the sun begins to set on my mothering years, when my boy has stretched upward and turned into a man, I’ll turn my eyes upward to meet his. I’ll try to puzzle together how this day has ended, how it’s already over, and I’ll ask, with a tightness in my throat and tears in my eyes, “Can we make this last just a little bit longer?”

Tonight, while he is still small, I will make it last longer. I’ll climb into his crib and hold him tightly as I read to him and try, with every fiber of my being, to make this season last as long as I can.